Thermally driven energy transfer systems (e.g. heat pumps, high temperature power units) are employed in a variety of energy conversion environments, from nuclear power generators to spacecraft. One example of a heat pump system, which uses a Rankine heat engine to power a vapor compression heat pump, is diagrammatically illustrated in FIG. 1 as comprising a boiler-driven turbine 11, the output shaft 13 of which drives a compressor 12. Each of turbine 11 and compressor 12 uses a common working fluid used which is collected by a condenser 15 and coupled therefrom to a boiler 17, via a pump 19, and an evaporator 14, via a throttling valve 16. By using a common working fluid for both the turbine and the compressor, these two units may be hermetically sealed in a single unit, shown by broken lines 20, thereby reducing the need for (periodically maintained) reservoirs of both lubricant (for the drive shaft 13) and refrigerant, and thus reducing system weight and complexity, factors which are of particular importance in spaceborne applications. Rankine powered compression systems which utilize a common condenser (the single working fluid approach) have performance and weight advantages over chemical or absorption heat pump systems. However, they still employ a rotating (turbine) or reciprocating expander and compressor, which are not only sybject to breakdown, but low exit qualities can cause turbine blade imbalance and blade erosion, resulting from the impingement of droplets on the turbine blades.
An additional factor in the operation of heat pump containing thermal energy transfer systems is the ability of the working fluid to perform successfully over the temperature range to which it is subjected in the course of its transit between heat source and heat sink. Its melting point must be lower than the lowest temperature in the heat exchange cycle in order to prevent fluid precipitation throughout the cycle and its critical temperature must be above the heat sink temperature, so as to allow the fluid to condense at the heat sink temperature. In addition, the fluid should not build up excessive pressure at the highest temperature of the cycle and should be stable at this temperature. Ideally, the working fluid should have as low a specific volume as possible, in order to minimize the volume of the system and it must be stable and compatible with the materials of which the system is constructed.
Currently, there are three principal types of working fluids in use for high-temperature power and heat pump systems: 1) high-molecular weight organic fluids; 2) liquid metals; and 3) water. Because high-molecular weight fluids can be used in cycles with temperatures only up to about 700K, they cannot be applied to systems that operates over a very wide temperature range, such as nuclear power generation plants, that have a temperature range on the order of 300 degrees K to 1500 degrees K, and spacecraft systems, having a power source temperature of 1000 degrees K and electronic components typically operating at 300 degrees K.
Nuclear power plants customarily handle the temperature range problem by using two working fluids in two cycles, using liquid metals in a range between the high end extreme and an intermediate temperature, and water between the intermediate temperature and the low end of the scale. Spaceborne systems, however, unlike terrestrial power plants, face a weight (payload) constraint in which the systems must be as lightweight and compact as possible and the fluid should have as low a specific volume as possible. Because the melting points of liquid metals are well above room temperature and their vapor pressures are too low at room temperature, the use of a liquid metal for the high end of the range would require a two fluid/two cycle system akin to that of a nuclear power plant, thereby adding unwanted complexity and weight to the system. Because of its thermal stability and its compatibility with many construction materials, water would appear to be a good candidate for a spaceborne application. Unfortunately, however, because of the high specific volume of steam, water cannot be employed satisfactorily at high temperatures. (It should be noted that common refrigerants, such as Freon (a registered Trademark of Dupont), are unstable at high temperatures and cannot be used in high temperature power cycles.)